Field at +q due to first dipole = $\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{2\mathrm{p}}{{\left(\mathrm{d}+\mathrm{l}\right)}^3}$

Force on +q due to first dipole = $\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{2\mathrm{pq}}{{\left(\mathrm{d}+\mathrm{l}\right)}^3}$
Force on -q due to first dipole = $\frac{1}{4{\mathrm{\pi \epsilon }}_0}\frac{2\mathrm{pq}}{{\left(\mathrm{d}-\mathrm{l}\right)}^3}$
Force of interaction = $\frac{2\mathrm{pq}}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{1}{{\left(\mathrm{d}-\mathrm{l}\right)}^3}-\frac{1}{{\left(\mathrm{d}+\mathrm{l}\right)}^3}\right]$
$=\frac{3{\mathrm{p}}^2}{2{\mathrm{\epsilon }}_0{\mathrm{\pi d}}^4}=2.1 \mathrm{x} {10}^{-16} \mathrm{N} \left[\because \mathrm{d}>>\mathrm{l}\right]$